I don't have the specific documentation for the '93, but the '94/'95 the cabriolet essentially uses a common controller for the seat belt extenders (unlike the coupe which has independent controllers), however, there are separate door switches and mechanisms for each side. Unless the two sides failed at different times or the car is new to you (previous maintenance history unknown), the odds of both mechanisms and/or door switches failing simultaneously are low, so it is best to look at the things that are common to both sides.
First, check fuses C and 1 (and when I say "check", I mean remove, clean, and replace, preferably with a new copper fuse) as these circuits power both extenders. Removing fuse C will also reset the state "memory" on the controller which is a good thing.
Second, check door switch - with the headlights on or the key in the ignition, do you get the warning buzzer? If so, at least the driver's door switch is likely working. If not, check the switch.
Third, Remove fuse C. With the car off, gently pull of the extender to see if you can withdraw it part way. Don't pull too hard, or you may pull the triangle off the arm. If it won't move, don't keep pulling. If you can move it replace fuse C (the mechanism may pull back in when you replace the fuse), open and close the door and start the car and see if the mechanism responds. If you can't move it, just replace fuse C.
If these tests fail and the two sides failed at the same time, I would suspect the control module. If the sides failed at different times, the mechanisms are the primary suspect.
Getting harder to find carbio parts these days, so I hope it's a simple fix.